1. Field of the Invention
The present invention relates to a pulse width modulation circuit, and more particularly to a pulse width modulation circuit composed of a general-purpose pulse width modulation circuit and adapted to operate at a high speed, and also an illuminating device using the circuit.
2. Description of the Related Art
A pulse width modulation circuit is used in various devices, and is commercially available as a general-purpose LSI (Large Scale Integration) recently. The specification of such a pulse width modulation circuit is determined for versatile applications. For example, referring to FIG. 5 showing a circuit block diagram, an LSI “FA13842 (Product Name)” 1 for a pulse width modulation circuit comprises a self-excited oscillator 3 which oscillates at a constant frequency determined by a resistor RT connected between terminals {circle around (8)} and {circle around (4)} of the LSI1 and by a capacitor CT having its one end connected to the terminal {circle around (4)} and the other end grounded, and the LSI 1 charges the capacitor CT by 5 V REF of the terminal {circle around (8)} via the resistor RT.
A circuit including two UVLOs (Undervoltage Lockout circuit) in the LSI 1 is adapted to check VCC VREF before an output stage (OUT terminal) starts operation. The circuit is not directly related to a pulse width modulation circuit which is the main subject of the present invention, so its operation is not described here. In FIG. 5, a MOS FET to drive current to an inductance is shown as an example of load, and an output terminal {circle around (6)} of the LSI 1 is connected to a gate of the MOS FET. And a resistor RS adapted to detect current has its one end connected to a source of the MOS FET and has its other end grounded. For ease of understanding, the pulse width modulation circuit will be described with reference to FIGS. 6 and 7A. FIG. 6 is a diagram of a conventional circuit to perform a pulse width modulation by using the LSI 1, and FIG. 7A is a time chart for explaining the operation. The LSI 1 comprises the aforementioned self-excited oscillator 3, an error amplifier 2, and a differential amplifier 4 as shown in FIG. 5, and is adapted to output a rectangular wave signal OUT with modulated pulse width as shown in FIG. 7A at an OUTPUT terminal {circle around (6)} according to a luminance signal (not shown) inputted from an outside source to the self-excited oscillator 3.
Referring to FIG. 6, the self-excited oscillator 3 oscillates at a constant frequency determined by the resistor RT and the capacitor CT. Specifically, in the LSI 1 (FA13842), the capacitor CT is charged up to about 3 V by DC 5 V from the VREF at the terminal {circle around (8)} via the resistor RT, and is discharged down to about 1.4 V by a discharging circuit in the self-excited oscillator 3. When the self-excited oscillator 3 is charged up to about 3 V, its output is boosted up to a high level, and the electrical charge stored in the capacitor CT is dissipated to go down to about 1.4 V by the discharging circuit of a resistor RT2 in the self-excited oscillator 3 thereby reducing the output of the self-excited oscillator 3 to a low level, which results in the self-excited oscillator 3 outputting a pulse signal SET shown in FIG. 7A.
The pulse signal SET thus generated is inputted to a set terminal of a flip-flop 5, and the flip-flop 5 is set by the falling edge of the pulse signal SET thereby increasing the signal OUT at the OUTPUT terminal {circle around (6)} up to a high level. When the signal OUT is outputted, the MOS FET shown in the figure turns on, and a current is caused to flow in an inductance of a load. The current generates a voltage at a resistor RS adapted to detect a current, and the voltage generated is inputted to a terminal {circle around (3)} as a detecting signal ISENS corresponding to the current. The signal ISENS has a same current waveform as produced when a rectangular waveform voltage is applied to the inductance and therefore generates a sawtooth voltage as shown in FIG. 7A. The signal ISENS is inputted to one input terminal {circle around (3)} of a comparator 4 and is compared with a signal COMP inputted to the other input terminal {circle around (1)} of the comparator 4. If the comparator 4 determines that the signal ISENS surpasses the signal COMP, the comparator 4 outputs a signal RESET at its output terminal, which resets the flip-flop 5 reducing the signal OUT at the output terminal {circle around (6)} to a low level. The signal COMP is a reference voltage to control the current flowing in the load at a predetermined value, specifically, when the load is a Xe lamp, is a luminance signal to apply a current to flow in the Xe lamp from outside to a COMP terminal {circle around (1)} of the LSI 1 in order to achieve a prescribed luminance. FIG. 7A shows the voltage of the signal COMP increases gradually. This is to indicate that the pulse width of the signal OUT varies according to the change of the voltage, that is to say a pulse width modulation is performed. When the voltage of the signal COMP increases, the pulse width of the signal OUT is increased thereby increasing a duty ratio. In other words, the current flowing in the Xe lamp increases, and thereby the luminance is enhanced. And the gradual increase of the signal COMP shown in FIG. 7 also indicates an example of a method to slow start lighting the Xe lamp, namely a starting method to gradually increase the luminance of the Xe lamp. In the slow start method, a signal COMP is applied as shown in FIG. 7A, whereby the current supplied to the Xe lamp is gradually increased so as to gradually enhance the luminance of the Xe lamp.
In the above described operation, the pulse signal SET, that is the output of the oscillator 3, stays at a low level for a time period until the capacitor is charged up to about 3 V. The charge voltage of the capacitor CT increases at a time constant determined by the resistor RT and the capacitor CT. In other words, the upper limit of the oscillation frequency of the self-excited oscillator 3 is determined by the resistor RT and the capacitor CT, and in the conventional circuit shown in FIG. 6 the oscillation frequency does not exceed the limit.
A discharge lamp (e.g., cold-cathode fluorescent lamp) has been increasingly used as a light source for a scanner in accordance with the spread of color photocopiers. In using the discharge lamp, it is demanded that the luminance be modulated to be optimized in consideration of the variance in the luminance of lamps and in the sensitivity of sensors, and the kind of manuscripts to be scanned. Conventionally, this luminance modulation has been generally performed by controlling the exposure time of a sensor, or by changing the lamp current as above described. For example, in the discharge lamp lighting device to light a cold-cathode fluorescent lamp (referred to also as “backlight inverter device”), the cold-cathode fluorescent lamp is lighted by an AC voltage and has its luminance modulated by changing the tube current. The tube current flowing in the cold-cathode fluorescent lamp is uninterrupted and is controlled to stay at a predetermined level, and the luminance of the cold-cathode fluorescent lamp can be modulated somewhere from 50% up to 100% maximum. This modulation method is generally called “current modulation method”.
As above described, the light source for a scanner is desired to be capable of having its luminance or illuminance modulated continuously and smoothly. In this regard, the method to control the tube current of the cold-cathode fluorescent lamp is convenient. The cold-cathode fluorescent lamp, however, has emission characteristics different from those of an incandescent lamp, and is not suitable for a color scanner. Also, it is difficult for the fluorescent lamp to achieve an enhanced luminance. Furthermore, in order to expand the area of a lower luminance side to thereby expand the modulation range, the tube current in the fluorescent lamp must be decreased. However, since a decrease in the tube current inherently leads to an unstable electrical discharge, the lower luminance side has its lowest luminance limited to about 50% of the maximum luminance in the continuous constant current control method above described, and also the relation between the tube current and the luminance is not linear thereby making the control difficult. Moreover, the cold-cathode fluorescent lamp has variance in its characteristics, making it difficult to achieve the optimum tube current.
And, when the aforementioned LSI “FA13842 (Product Name)” is used for a pulse width modulation circuit in order to light the discharge lamp at a high frequency, there is a problem that the LSI has an upper limit in oscillation frequency and may not appropriately work depending on the kind of a discharge lamp to be lighted. Conventionally, the oscillation frequency of the LSI has been increased by such a method that a transistor TRI is connected to both ends of the capacitor CT via a resistor r1 at one end as shown in FIG. 6, and a PCT1 synchronized is applied to a base of the transistor TR1 when the transistor TR1 is discharged thereby reducing the discharge time. In this connection, the resistor r1, which is a protective resistor to prevent a current exceeding a carrying capacity from flowing in the transistor TR1, may be omitted. However, there is a limitation to the increase in the oscillation frequency.